environment.rb

#...

ActionView::Base.field_error_proc = Proc.new do |html_tag, instance|
  if html_tag =~ /<label/
    %|#{html_tag} <font class="error">#{[instance.error_message].join(', ')}</font>|
  else
    error_class = 'fieldWithErrors'
    nodes = Hpricot(html_tag)
    nodes.each_child do |node|
      unless !node.elem? || node[:type] == 'hidden' || node.classes.include?(error_class)
        node[:class] = node.classes.push(error_class).join(' ')
      end
    end
    nodes.to_html
  end
end